The present invention relates to sheet feeding apparatus and in particular to a tray on which sheets are stacked for use with a bottom vacuum corrugating feeding apparatus.
With the advent of high speed xerographic copy reproduction machines wherein copies can be produced at a rate in excess of three thousand copies per hour, the need for a document handler to feed documents to the copy platen of the machine in a rapid, dependable manner was recognized to enable full utilization of the reproduction machine's potential copy output. A number of document handlers are currently available to fill that need. These document handlers must operate flawlessly to virtually eliminate the risk of damaging the originals and generate minimum machine shutdowns due to uncorrectable misfeeds or document multifeeds. It is in the initial separation of the individual documents from the document stack where the greatest number of problems occur.
Since the documents must be handled gently but positively to assure separation without damage through a number of cycles, a number of separators have been suggested such as friction rolls or belts used for fairly positive document feeding in conjunction with a retard belt, pad, or roll to prevent multifeeds. Vacuum separators such as sniffer tubes, rocker type vacuum rolls, or vacuum feed belts have also been utilized.
While the friction roll-retard systems are very positive, the action of the retard member; if it acts upon the printed face can cause smearing or partial erasure of the printed material on the document. Wih single sided documents, this does not present a problem as the separator can be designed so that the retard mechanism acts upon the underside of the document. However, with documents printed on both sides, there is no way to avoid the problem. Additionally, the reliable operation of friction retard feeders is highly dependent on the relative frictional properties of the paper being handled. This cannot be controlled in a document feeder.
In addition, a typical vacuum separating and feeding system is that described in Ser. No. 81,594 entitled "Sheet Separator" to Hamlin, wherein a plurality of friction belts is arranged to run over a vacuum plenum placed at the bottom of a sheet supply tray which has a "U" shaped pocket formed in it. The pocket serves to provide space for the bottom sheet to be captured by the vacuum feed belt assembly, to provide an air seal between the bottom document and the edges of the pocket and to provide a high pressure seal between the bottom sheet and the remainder of the stack. This high pressure seal is achieved by supporting a major portion of the stack weight on the edge regions of the pocket. This seal serves to convert the velocity energy of the air knife flow into a lifting pressure over the pocket area to levitate the remainder of the stack of sheets. This configuration has been used on a commercial scale in the Xerox 5600 machine and while it has been highly successful in operation, certain aspects can be improved. The "U" shaped or parabolic arc pocket configuration does not permit deformation of the sheet in a geometrically developable shape. Instead, it buckles the sheet around the edges and provides a leakage path for air to escape from the interface between the bottom sheet and the rest of the stack of sheets. This results in a reduction in the degree of levitation of the stack. Because of the shape of the parabolic arc the pressure distribution under the stack is not as efficient as could be and therefore, for a given degree of levitation higher air knife pressure is necessary which results in larger capacity air blowers and more noise. Finally, the parabolic arc shape does not adjust to different size paper since it works efficiently only over the parabolic pocket. In particular, difficulty is sometimes encountered in trying to levitate a stack of sheets substantially larger than the parabolic pocket.